In fiber optic communications, dense wavelength division multiplexing (DWDM) is a technique for multiplexing multiple optical carrier signals onto a single optical fiber. This form of frequency division multiplexing is commonly referred to as DWDM when applied to optical systems that employ a high level of multiplexing. The potential of optical fiber is more fully exploited when multiple beams of light at different frequencies (wavelengths) are transmitted on the same fiber. By using different wavelengths of laser light to carry different signals, capacity is multiplied. In a DWDM system, a multiplexer is used at the transmitter to join the signals together and a de-multiplexer is used at the receiver to split the signals apart.
An optical ring resonator is a device that is capable of both multiplexing and de-multiplexing, and it can function as an add-drop multiplexer on a fiber-optic communication bus. Optical ring resonators include a waveguide in a closed loop, coupled to one or more input/output (or bus) waveguides. When light of the appropriate wavelength is coupled from an input waveguide to the ring, constructive interference causes a buildup in intensity over multiple round-trips through the ring. The light is ultimately coupled to an output waveguide. Since only selected wavelengths resonate in the ring, the ring functions as a filter. A range of applications such as optical switching, electro-optical switching, wavelength conversion, and filtering have been demonstrated using optical ring resonators.
Conventional multiplexer switches integrated with wavelength selective devices, such as micro-electromechanical mirrors (MEMS), liquid crystal mirrors or lenses, and thermo-optic switches are typically limited to switching speeds in the millisecond range; even the fastest response ferroelectric liquid crystals and smallest thermo-optic switching speeds are limited to several microseconds. Although these components are suitable for configurable DWDM circuit networks, they are not suitable to replace electrical cross-connect switches used for network applications requiring very rapid (e.g., less than one microsecond) switching speeds, such as applications for tactical aircraft avionics. These relatively slow switches are also unsuitable for use in DWDM optical burst or optical packet switch based networks.
Existing technologies for wavelength selective devices: (1) switch too slowly; (2) only switch over a very limited wavelength range, namely a single full width at half maximum (FWHM); (3) are not readily compatible with silicon processing; (4) because of their large size due to low refractive index contrast and incompatibility with silicon processing, do not readily scale in the manner of a VLSI implementation; (5) are limited to configurable DWDM circuits; (6) do not readily extend over a 20 nanometer free spectral range required for colorless wavelength switching across the C-band; and (7) do not operate at practical voltages since electro-optic coefficients are insufficient and electrode gaps are too large.